The first Half-Life title has moved into the realm of the legendary when it comes to video games, and in some ways that sort of canonization has almost diminished how important the game continues to be, and how much we can learn from its design. People seem to think games this good, and arguably this important, just kind of happen; as if game design came from a lightning strike and left just as fast. The video embedded in this post is a great way to break free of that illusion.
The reality is that many very smart people decided that Half-Life was going to convey information to the player in some very interesting ways, and allow some interactions and world building that are rare even today. It's shocking how little Half-Life as a series has been exploited; the game hasn't been annualized and there are few quotes from it that kick around the popular vernacular. If anything the best lessons from the game remain unlearned by the rest of the industry, and it's a shame.
This 20 minute video does a great job of explaining why Half-Life continues to be so brilliant. It operates as a great reminder of what the game does well to those of us who haven't played in years, and it also works as a general primer for the people who may have never have played.
And if you fall into the latter category, I'd consider playing the game to be a homework assignment. The game was amazing for its time, and the best aspects of its design are still relevant.